Yonggang Tang

Magneto-Electric Properties of LaMnO3 with Monovalent Na Element Doping at A Site

Abstract By measuring M-T curves and R-T curves of the samples La 1- x Na x MnO 3 ( x =0.05, 0.10, 0.15, 0.20, 0. 25, 0.33), the influence of Na doping at La site on the magneto-electric properties was investigated. Results show that, Curie temperature T C increases and paramagnetic-ferromagnetic transition becomes steeper with increasing of Na content; all samples exhibit a paramagnetic-ferromagnetic transition associated with metal-insulator transition whereas the values of resistance decrease with the doping concentration increasing. The change of the magneto-electric properties originates from double-exchange function in Mn 3+ -O-Mn 4+ and static Jahn-Teller distortion as well as the change of tolerance factor caused by increasing of the doping concentration.

Magnetoresistance enhancement and temperature stability of magnetoresistance in La1−x(Sr1−yNay)xMnO3

A series of the samples La1−x(Sr1−yNay)xMnO3 (y=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by the solid-state reaction method. Magnetoresistance enhancement and temperature stability of magnetoresistance in the system La1−x(Sr1−yNay)xMnO3 with unchanged Mn3+/Mn4+ ratio through the doping of both monovalent and divalent elements at A site were studied through the measurements of X-ray diffraction (XRD) patterns, resistivity-temperature (ρ-T) curves and magnetoresistance-temperature (MR-T) curves. The results indicate that with the increase of Na doping amount, the peak value of MR increases, and it increases from 12.4% for y=0.2 to 50.6% for y=1.0 in the magnetic field B=0.8 T; ρ-T curves exhibit the double-peak phenomenon, which comes from the competition between the resistivity of surface phase and that of body phase; for the sample of y=0.8, MR increases slowly from 8.3% to 9.4% in the temperature range from 259 to 179 K, and MR is so stable in such a wide temperature range, which provides reference for the research on the temperature stability of MR.

Low-field magnetoresistance of (1−x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) composite system under different sintering temperatures of matrix

A series of (1−x)La0.6Dy0.1Sr0.3MnO3/0.5x(Sb2O3)(x=0.15) samples were prepared by the solid-state reaction method, and the influence of sintering temperature of the matrix on low-field magnetoresistance of (1−x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) was studied through the measurements of X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) image, resistivity-temperature (ρ−T) curves, and magnetoresistance-temperature (MR-T) curves. The results indicate that for the samples with low sintering temperature of the matrix, low-field magnetoresistance effect appears on the whole temperature range and can be explained by grain boundary effect; for the sample with high sintering temperature of the matrix, intrinsic magnetoresistance peak appears on the high-temperature range, low-field magnetoresistance effect appears on low temperature range, and the magnetoresistance in the magnetic field of 0.2 T and on the comparatively large temperature range between 280 K and 225 K hardly changes with temperature and remains at 4.8%, which can be explained by the competition between the intrinsic magnetoresistance induced by double-exchange function inside grains and the tunneling magnetoresistance (TMR) induced by grain boundary effect. The temperature stability of magnetoresistance is beneficial to the practical applications of MR.